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Car Library

Plain-language reference for the parts and concepts behind every repair. No ads, no upsells, no marketing fluff — just clean explanations of what each thing actually is and how to think about it.

Engine basics

Timing belt vs timing chain

A timing belt is a toothed rubber belt that synchronizes the camshaft with the crankshaft. It's quieter, lighter, and used on many older or budget engines (Honda J-series V6, Subaru EJ25, Toyota 1MZ-FE). Timing belts must be replaced on a schedule, typically every 60,000–100,000 miles, or the belt will eventually break and cause severe engine damage on interference engines. A timing chain is a metal chain doing the same job. Chains are heavier and slightly noisier but designed to last the life of the engine — no scheduled replacement. Most modern engines (Toyota 2GR-FE, Honda K-series, BMW M50/52/54/N52, Subaru FA/FB-series) use chains. Some engines have hybrid systems (chain-driven primary cams + belt-driven secondary). To check yours, look up your specific engine code at openlaborproject.com.

OHV vs OHC vs DOHC

OHV (overhead valve) engines have the camshaft in the engine block, with pushrods opening valves in the cylinder head — common on classic American V8s (Chevy small-block, Ford 302, Dodge 318). OHC (overhead cam) puts the camshaft in the head itself, eliminating pushrods. SOHC has one camshaft per head; DOHC has two (one for intake valves, one for exhaust). DOHC enables four valves per cylinder for better breathing — standard on most modern performance engines. OHV is simpler and more compact; OHC/DOHC is more efficient and supports higher RPM.

Engine displacement

The total volume swept by all the engine's pistons through one full cycle. Measured in liters (L), cubic centimeters (cc), or cubic inches (ci). A 2.5L engine has 2,500 cc of swept volume. 1 liter ≈ 61 cubic inches, so a 350 ci small-block Chevy ≈ 5.7L. Bigger displacement generally means more torque at low RPM but worse fuel economy. Forced induction (turbo or supercharger) lets a smaller engine make the power of a larger one — a turbocharged 2.0L can match a naturally-aspirated 3.5L V6 in real-world output.

Turbocharger vs supercharger

Both compress intake air to force more into the cylinders, increasing power output. A turbocharger uses exhaust gas to spin a turbine that drives a compressor — efficient because it recycles otherwise-wasted exhaust energy, but suffers from "turbo lag" while exhaust pressure builds. A supercharger is mechanically driven by the engine (belt or gear), giving instant boost but at the cost of power consumed to drive it. Turbos are far more common on modern engines because they're more fuel-efficient. Twin-turbo and twin-scroll designs reduce lag.

Compression ratio

The ratio between cylinder volume at bottom-dead-center (piston down) and top-dead-center (piston up). Typical naturally-aspirated engines run 9.5:1 to 11:1; turbocharged engines run lower (8.5:1 to 10:1) because boost effectively raises the dynamic compression. Higher compression = more efficiency and power, but requires higher-octane fuel to avoid knock. Diesel engines run very high compression (16:1 to 22:1) — they ignite by compression alone, no spark plug.

Drivetrain

FWD, RWD, AWD, and 4WD

FWD (front-wheel drive): engine drives the front wheels only. Cheaper to build, better traction in snow, eats inner CV axles. RWD (rear-wheel drive): drives the rear wheels. Better for performance and towing; standard on most trucks, sports cars, luxury sedans. AWD (all-wheel drive): permanent or on-demand power to all four wheels via a center differential. Modern AWD systems shuffle torque dynamically. 4WD (four-wheel drive): typically a part-time system with a transfer case — driver selects 2HI, 4HI, 4LO. 4WD locks front and rear axles together for off-road traction; not designed for use on dry pavement (causes binding).

Transfer case

A gearbox that splits power from the transmission to both the front and rear differentials in 4WD vehicles. Has its own fluid (typically a special transfer-case fluid like Mopar AS68RC or Toyota T-IV — check your manual). Failures often show up as a chatter or grinding when shifting between 2HI and 4HI, or fluid leaks. Most transfer cases hold 1.5–2.5 quarts.

Limited-slip differential (LSD)

A differential that allows wheels to spin at different speeds (necessary for cornering) but limits how much they can differ. Without LSD, an open differential sends all torque to the wheel with the least traction — useless on ice or in mud. LSD types: clutch-pack (Trac-Lok, Posi), gear-driven (Torsen, Eaton TrueTrac), viscous-coupling, electronic (ESC-based brake-clutching). LSD fluid is often specified separately from regular differential gear oil — using the wrong fluid can cause chatter on turns.

CV axle

Constant-velocity axle: the front (and sometimes rear) drive shafts on FWD and AWD vehicles. CV joints allow the axle to flex with steering and suspension travel without losing power. Outer CV joints (closer to wheel) usually fail first; symptom is a clicking noise during tight turns. Inner CV joints (closer to transmission) failing usually shows up as a vibration under acceleration. Replacement labor: typically 1.5–3.0 hours per side depending on vehicle.

Diagnostics

OBD-II (on-board diagnostics)

Federally mandated on all gasoline vehicles sold in the US since 1996, diesel since 1997. OBD-II is a standardized diagnostic system with a 16-pin connector (typically under the driver's dash) that any scan tool can read. It monitors emissions-related systems and stores Diagnostic Trouble Codes (DTCs) when something fails. The check engine light (also called the malfunction indicator lamp / MIL) illuminates when a code is set. Codes are five characters: a letter (P, B, C, U) plus four digits.

Diagnostic Trouble Codes (DTCs)

Five-character alphanumeric codes set by the vehicle's onboard diagnostics when a fault is detected. P codes = powertrain (engine, transmission, fuel, emissions); B codes = body (airbags, climate control); C codes = chassis (ABS, steering); U codes = network/communication. P0XXX codes are generic across all manufacturers; P1XXX codes are manufacturer-specific. Example: P0301 = "Cylinder 1 misfire detected" (generic). Reading codes requires an OBD-II scan tool — basic Bluetooth dongles cost $20, professional tools (Snap-On, Autel) run $1,000+. Look up specific codes at openlaborproject.com/dtc-codes.

Freeze frame data

When a DTC is set, the ECU snapshots a set of sensor readings at that exact moment — engine RPM, coolant temp, fuel trim, vehicle speed, calculated load, etc. This freeze frame survives even after the code is cleared and is invaluable for diagnosing intermittent faults. A misfire code at 78°F coolant and 2,200 RPM with -15% fuel trim tells a different story than one at 195°F and idle. Most scan tools display freeze frame alongside the code.

Short-term and long-term fuel trim

The percentage adjustment the ECU is making to baseline fuel injection to maintain stoichiometric air/fuel ratio (14.7:1 for gasoline). Short-term fuel trim (STFT) reacts within seconds to oxygen sensor data; long-term fuel trim (LTFT) is the slow-moving average the ECU uses as its baseline. Healthy ranges: ±5%. Persistent positive trim (+10% or more) means the ECU is adding fuel — likely a vacuum leak, MAF sensor problem, or low fuel pressure. Persistent negative trim means the ECU is pulling fuel out — likely a leaking injector or contaminated MAF.

Brakes

Brake pads and rotors

Brake pads are the friction material that clamps onto the rotor (disc). Pad types: organic (quiet, soft, more dust), semi-metallic (durable, more noise, more dust), ceramic (clean, quiet, premium price). Rotor types: solid (rear, light-duty), vented (front, dissipates heat), drilled/slotted (performance, vents gas from pad outgassing). Replacement intervals vary wildly — pads can last 25K–80K miles depending on driving style and pad type. Front pads wear faster than rear because of weight transfer under braking. Rotors should typically be replaced or resurfaced when pads are replaced.

ABS (anti-lock braking system)

Pulses brake pressure 15+ times per second when wheel speed sensors detect imminent lockup, allowing the driver to maintain steering control during hard braking. ABS doesn't necessarily shorten stopping distance on dry pavement — its purpose is steering control, not max deceleration. The ABS module is typically a separate hydraulic unit with its own electronic control. Common failure modes: wheel speed sensor (often after a hub bearing replacement disturbs the sensor wire), ABS pump motor (older modules), or hydraulic valve sticking. ABS faults set C-codes.

Brake fluid types (DOT 3, 4, 5, 5.1)

DOT 3 and DOT 4 are glycol-based and hygroscopic (absorb water from air). DOT 4 has a higher boiling point — required on many European vehicles. DOT 5 is silicone-based and incompatible with DOT 3/4 — used in military and some classic cars; never mix with glycol fluid. DOT 5.1 is glycol-based like 3/4 (despite the higher number) with a very high boiling point — used in performance applications. Brake fluid should be flushed every 2 years or 30,000 miles regardless of how the brakes feel — water contamination drops the boiling point and corrodes ABS components.

Brake caliper

The hydraulic clamp that squeezes brake pads against the rotor. Floating calipers (most common) have one piston on the inboard side and float on slide pins; the entire caliper body shifts when pressure is applied. Fixed calipers (performance vehicles) have pistons on both sides — better feel, higher cost. Common failures: stuck slide pins (pad wear becomes uneven), seized piston (caliper drags, brake gets hot), torn dust boots (water gets in, piston rusts). A dragging caliper can wear pads completely on one side while the other side looks new.

Fluids & maintenance

Engine oil viscosity (SAE grades)

Engine oil viscosity is rated by SAE — for example, "5W-30." The number before W is the cold-weather (winter) viscosity rating; the second number is the operating-temperature viscosity. Lower W numbers flow better when cold (5W flows better at startup than 10W). Higher second numbers protect better at high temperatures. Use the viscosity specified by the manufacturer — modern engines have tight tolerances designed around specific oil grades. Synthetic oil flows better when cold, lasts longer, and resists thermal breakdown — required on most engines built since 2010.

Oil change intervals

The "every 3,000 miles" rule is decades out of date. Modern synthetic oils paired with modern engines safely run 7,500–10,000 miles between changes; some vehicles with oil life monitors recommend 12,000+ miles. Always follow the manufacturer's recommended interval in your owner's manual. Severe service conditions (short trips, dusty environments, towing, extreme temperatures) shorten the interval. Don't go by oil color — modern detergent oils turn dark within a few hundred miles, and that's the additive package working as designed, not a sign the oil is "dirty."

Transmission fluid (ATF)

Automatic transmission fluid is engineered specifically for each transmission family — Dexron VI, Mercon LV, ATF+4, Toyota WS, Honda DW-1, etc. are not interchangeable. Using the wrong fluid causes shift quality issues, internal damage, or slipping. CVT (continuously variable transmission) fluids are entirely different and 100% specific to each manufacturer's CVT. Manual transmissions typically use gear oil (75W-90, 80W-90) or a manufacturer-specific MTF. Most modern automatics are advertised as "lifetime fill" — but most experts recommend a fluid service every 60,000–100,000 miles regardless.

Coolant types (IAT, OAT, HOAT)

Coolant chemistry varies by manufacturer. IAT (inorganic additive technology, older green coolant) is largely obsolete — used in pre-2000 American cars. OAT (organic acid technology) — Dex-Cool (orange, GM), Toyota pink/red, longer-life. HOAT (hybrid OAT) — Ford yellow, Chrysler MS-12106. Mixing types causes precipitates and silicate dropout that clog radiators. Always use the coolant specified for your vehicle. Coolant should be flushed every 5 years or 100,000 miles even if it still looks clean — additive depletion is invisible but real.

Battery & electrical

CCA (cold cranking amps)

The number of amps a battery can deliver at 0°F for 30 seconds while maintaining at least 7.2 volts. The standard rating used to size automotive batteries. A vehicle's required CCA depends on engine displacement, climate, and accessory load — typical passenger car: 500–800 CCA; diesel truck: 800–1,000+. Always meet or exceed the manufacturer's spec; going lower causes hard starts in cold weather. CCA naturally drops as a battery ages — a battery rated 700 CCA may only deliver 400 after 4 years.

AGM battery

Absorbed Glass Mat — a sealed lead-acid battery design where the electrolyte is held in fiberglass mats between the plates instead of free-floating. AGM batteries handle deep discharge cycles better, last longer, don't leak when tipped, and tolerate vibration. Most start-stop and hybrid vehicles require AGM because of the deep-cycle demand of restarting the engine constantly. AGM batteries cost about 2x a standard flooded battery but typically last 50% longer. Charging requires AGM-aware chargers — overcharging damages them.

BCI group size

The Battery Council International standard for battery physical dimensions and terminal placement. Common sizes: Group 24, 27, 31 (passenger cars and light trucks); Group 35 (Japanese imports); Group 65 (full-size domestic cars and trucks); Group 78 (side-terminal). The group size must match what fits your vehicle's tray and reach the cables. Look up your exact group at openlaborproject.com/battery-specs.

Alternator

The engine-driven generator that powers the electrical system and recharges the battery. Output is regulated by an internal voltage regulator (target: 13.8–14.7V at the battery while running). Common failures: worn brushes (bearings whine, output drops), bad voltage regulator (overcharging or undercharging), shorted diode (AC ripple, drains battery overnight). Symptoms of a failing alternator: dim headlights at idle, battery warning light, accessory glitches. Charging system testing requires a load tester and a meter — don't condemn the alternator without testing the battery first.

HVAC & A/C

Refrigerant R-1234yf vs R-134a

R-134a was the standard automotive A/C refrigerant from 1995 until ~2017. R-1234yf replaced it in new vehicles starting around 2014–2018 (mandated by EPA for lower global warming potential). The two are NOT interchangeable — different oils, different pressures, different fittings, different leak detection. Mixing them causes compressor damage. R-1234yf is significantly more expensive to charge ($150+ vs $30 for R-134a). Always check what's specified on the under-hood A/C label before opening the system.

A/C compressor

Pumps refrigerant through the A/C system, with an electromagnetic clutch that engages and disengages based on pressure and demand. Common failure modes: clutch bearing wear (squealing noise that comes and goes with A/C use), internal seal failure (refrigerant leak), full mechanical seizure (locks the accessory belt). When a compressor fails internally, the entire system must be flushed, the receiver/dryer or accumulator replaced, and proper oil charge re-established — never just bolt on a new compressor without flushing.

Tires & alignment

Tire pressure (PSI)

Always use the pressure on the door jamb sticker, not the max pressure printed on the tire sidewall. The sidewall number is the maximum the tire can hold structurally, not the recommended setting. Underinflation causes excessive sidewall flex, heat buildup, and outer-edge wear; overinflation causes a harsh ride and center-tread wear. Check pressure cold (sitting overnight) — driving heats tires and raises pressure 3–5 PSI artificially. TPMS (tire pressure monitoring system) is mandatory on all US passenger vehicles since 2008.

Wheel alignment (camber, caster, toe)

Camber is the inward/outward tilt of the wheel viewed from the front (negative = top tilts in). Caster is the forward/backward tilt of the steering axis (positive caster gives stability at speed). Toe is whether the wheels point straight, in (toe-in), or out (toe-out). Misalignment shows up as uneven tire wear: inside-edge wear suggests excess negative camber or toe-out; outer wear suggests positive camber or toe-in. After hitting a hard pothole or a curb, get an alignment check — even a slight bend can wear an expensive tire to nothing in a few thousand miles.

Want a specific term covered?

Email [email protected] with the term and we'll add it. The library grows as our user community asks for it.

For vehicle-specific specs (labor times, torque values, fluid capacities, DTC codes, battery specs), use the search on the homepage — every vehicle has its own dedicated page.